Dendrimers are branched macromolecules with a core and attached dendrons, also known as dendrites. Dendrons are branched structures comprising branching units and optionally linking units. The generation of a dendron is defined by the number of sets of branching groups; see FIG. 1. Dendrons with the same structure (architecture) but a higher generation, or order, are composed of the same structural units (branching and linking units) but have an additional level of branching. There can be surface groups on the periphery or distal units of the dendrons.
Dendrimers of different generations can have different types of branching points and linking units. Dendrimers are generally synthesised by convergent or divergent routes. Convergent routes require a functional group at the dendron foci which can either be reacted directly to give a higher generation dendron or dendrimer or activated before the reaction to form a higher generation dendron or dendrimer. For divergent routes the distal functional groups are used, either after activation or directly, to form the next higher generation dendrimer. Once the dendrimer is formed it has been demonstrated that the surface groups can be modified, e.g. t-butylcarbonates can be removed to leave hydroxyl moieties. The linking, branching, and core units of a dendrimer can either be made up of saturated or unsaturated units. The presence of unsaturated units within a dendron or dendrimer gives rise to the possibility of modifying the structure to form a final dendrimer that has beneficial properties and which could not be formed easily by another method. In particular this invention pertains to a chemical conversion of one or more unsaturated units within a dendron or dendrimer to give bonds which are more saturated between the atoms within the unit. The process is different from other reported reactions of unsaturated units. For example it has previously been shown that when di-substituted vinylene units are present within a dendrimer they can be isomerised (see J. N. G. Pillow et al, Macromolecules, 1999, 32, 5985). Although this is a chemical transformation it does not change the level of saturation and therefore falls outside the scope of this invention. In addition, it has been reported that phenylene based dendrimers have been oxidised to form graphite like structures (FIG. 8A illustrates one component of a dendrimer) (M. D. Watson et al, Chem. Rev., 2001, 101, 1267). Although this is a reaction within the dendron and/or dendrimer structure it does not constitute a reduction in the level of saturation of the sp2 hybridised carbons of the benzene rings. In this case the carbon-proton bond is merely converted to a carbon-carbon bond. Similarly di-dendroned substituted 4,4′-diphenylacetylene dendrons have been cyclised to form larger benzene centered dendrimers (M. D. Watson et al, Chem. Rev., 2001, 101, 1267). The starting material in FIG. 8B is a dendron where the acetylene unit is the focus with two dendrons attached. The foci of three of these dendrons react to form the central benzene unit of the dendrimer and the other components of the dendrons are unchanged. Recently dendrimers that have unsaturated units have been shown to be useful as the light emitting-layer in organic light-emitting diodes.
Organic light-emitting diodes (OLEDs), also known as organic electroluminescent (EL) devices, are an emerging display technology. In essence an OLED comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied visible or other light is emitted. At least one of the electrodes must be transparent to light. For display applications the light must of course be visible to the eye, and therefore at least one of the electrodes must be transparent to visible light.
There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. Significant work has been undertaken to develop appropriate materials, particularly polymers. More recently dendrimers that are photoluminescent in the solid state have been shown to have great promise as solution processible light-emitting materials in OLEDs (S.-C. Lo, et al Adv. Mater., 2002, 13, 975; J. P. J., Markham, et al Appl. Phys. Lett., 2002, 80, 2645).
Light-emitting dendrimers typically have a luminescent core and in many cases an inherently at least partially conjugated dendrons. As used herein, an inherently at least partially conjugated dendritic structure is one in which there is conjugation between the groups making up the dendritic structure, but the pi-system is not necessarily fully delocalised. The delocalisation of the pi-system is dependent on the regiochemistry of the attachment of the different groups. Such dendrons can also be conjugated dendrons. Further examples of light-emitting dendrimers include those found in P. W. Wang, et al Adv. Mater., 1996, 8, 237; M. Halim, et al Adv. Mater., 1999, 11, 371; A. W. Freeman, et al J. Am. Chem. Soc., 2000, 122, 12385; A. Adronov, et al Chem. Comm., 2000, 1701; C. C. Kwok, et al Macromolecules, 2001, 34, 6821. Light-emitting dendrimers have the advantage over light-emitting polymers that the light-emitting properties and the processing properties can be independently optimised as the nature of the core, dendrons and surface groups can be independently altered. For example with dendrimers that contain light-emitting cores the emission colour of the dendrimer can be changed by simply changing the core. Although dendrimers with a light-emitting core are preferred, when the core is not luminescent the chromophores in the dendron can be light-emitting.
Other physical properties, such as viscosity, may also make dendrimers more easily tailored to the available manufacturing processes than polymers. Organometallic dendrimers have previously been used in OLED applications as a single component in a film (i.e. a neat film) or in a blend with a molecular material or in a blend of more than one dendrimer of different type (i.e. different cores), e.g. J. M. Lupton et al. Adv. Funct. Mater., 2001, 11, 287 and J. P. J., Markham, et al Appl. Phys. Lett., 2002, 80, 2645.
Intermolecular interactions play an important role in the opto-electronic properties of organic light-emitting and transport materials. Close contact and good order can lead to high charge mobilities but can also give rise to reduced emission due to the formation of excited-state dimers. In previous work we have shown that intermolecular interactions can be controlled by the generation of the dendrons attached to a dendrimer (J. M. Lupton, et al Phys. Rev. B, 2001, 63, 5206; J. P. J., Markham, et al Appl. Phys. Lett., 2002, 80, 2645). However the nature of the intermolecular interactions is affected by the type of dendron that is attached to the core. Generally, when the dendrimers are prepared via a convergent or divergent route the main structure of the final dendrimer is defined by the dendron branching groups and linking units used in the synthesis. This gives a limitation over the control of the dendrimer architecture and properties. Within a dendron or dendrimer there is potential for the surface groups, branching groups and linking units, where present, and foci in the case of dendrons, and core in the case of dendrimers, to be modified. It is known that surface groups of dendrimers and dendrons and the foci of dendrons can be modified. However, as many dendrimers contain saturated linking units and branching groups the modification of these units can be difficult. In contrast dendrons or dendrimers containing unsaturated units within the linking units and branching groups of the dendrons and the core offer an unexpected advantage for modifying the dendron and dendrimer structures. This is different from the reactions used in the divergent or convergent route where a surface or focal group that contains unsaturation is converted into a surface group for further generation building. For example, in a divergent route the surface groups of the lower generation are typically activated to make them reactive to the species added in the next stage. This is illustrated in the synthesis of poly(iminopropane-1,3-diyl) (PPI) dendrimers which are formed by the Michael addition of acrylonitrile to 1,4-diaminobutane. After the addition the resultant terminal nitrile groups are reduced to form the primary amine terminated first generation dendrimer, which can then be reacted with more equivalents of acrylonitrile. The reduction is repeated to form the next generation dendrimer (Topics in Current Chemistry, Dendrimers III, p 86, Springer-Verlag, Berlin Heidelberg, 2001). This approach is different from that of the current invention, in which the entire dendron or dendrimer structure is built such that it contains unsaturated units within the branching groups and/or linking units and/or core, and then the dendron or dendrimer is modified by reaction of some or all of said units. In particular unsaturated groups such as acetylenyl and vinyl groups within the dendron of a dendrimer are reacted. With this method we have been able to produce dendrimers with aryl branching groups linked by ethylene units using the powerful Pd catalysed coupling of aryl halides and alkenes, followed by simple hydrogenation. In contrast, previous routes to saturated links between aromatic branching groups have involved a complex series of reactions where the hydrocarbon linking unit is introduced via a number of steps prior to the Pd coupling to form the dendrimer (Z. Bo, A. D. Schlüter J. Org. Chem. 2002, 67, 5327). A further advantage of this invention is that it gives additional flexibility in the choice of end product. After the coupling to form the unsaturated unit a range of reactions, such as halogenation and hydrogenation of alkenes and acetylenes can be used to give a variety of alternative products from the same intermediate material. We have discovered that by using dendrons that contain acetylene and vinylene links between branching points in dendrons and further reacting them it gives a way of advantageously modifying the dendrons and hence the dendrimers.